Troubleshooting with Insulation
Resistance Test Instruments
INTRODUCTION
DC voltage to be applied during an insulation resistance test.
In order to obtain meaningful insulation resistance measurements, the technician should carefully examine the system
under test. The best results are achieved
when the following conditions are met:
A troubleshooting procedure consists of
systematic testing designed to identify
and correct a problem. Technicians must
gather information and utilize tests to
identify problems and develop appropriate solutions. Following established
troubleshooting procedures ensures that
problems are identified and quickly corrected to minimize equipment and production downtime. Consistent practices
also generate consistent measurements
for more accurate comparison and tracking over time.
• The system or equipment is taken out
of service and disconnected from all
other circuits, switches, capacitors,
overcurrent protection devices, and
circuit breakers. Ensure that the measurements are not affected by leakage
current through switches and overcurrent protective devices.
For accurate low-resistance measurements
when using a DMM, insulation multimeter,
or megohmmeter, the resistance of the test
leads is subtracted from the total resistance.
Typical test lead resistance is between 0.2 Ω
and 0.5 Ω.
• The temperature of the conductor is
above the dew point of the ambient air.
When this is not the case, a moisture
coating will form on the insulation
surface, and, in some cases will be
absorbed by the material.
TROUBLESHOOTING OF
ELECTRIC MOTORS
• The surface of the conductor is free of
hydrocarbons and other foreign matter
that can become conductive in humid
conditions.
Insulation resistance testing is performed
when troubleshooting electric motors
and related equipment. IEEE Standard
43-2000, Recommended Practice for
Testing Insulation Resistance of Rotating
Machines, recommends the insulation
test voltage to apply, based on winding
rating, and minimum acceptable values
for electric motor windings. The IEEE
also provides typical guidelines for
• Applied voltage is not higher than the
system capacity. When testing lowvoltage systems, too much voltage can
overstress or damage insulation.
• The system under test has been completely discharged to the ground. The grounding discharge time should be about five
times the testing charge time.
21
22
INSULATION RESISTANCE TESTING
• The effect of temperature is considered. Since insulation resistance is
inversely proportional to insulation
temperature (resistance goes down as
temperature goes up), the recorded
readings are altered by changes in the
temperature of the insulating material. It is recommended that tests be
performed at a standard conductor
temperature of 68°F (20°C).
When comparing readings to 68°F
base temperature, double the resistance
for every 18°F (10°C) above 68°F or
halve the resistance for every 18°F below 68°F in temperature. For example,
a 1 MΩ resistance at 104°F (40°C)
will translate to 4 MΩ resistance at
68°F (20°C).
To measure the temperature of motor
windings, use a noncontact temperature
probe connected to a digital multimeter
(DMM) or an infrared thermometer.
See Figure 3-1.
Although insulating materials in practical
use can be solid, liquid, or gas, resistance
measurements generally refer to solid
insulation. Factors that affect insulation
resistance measurements are the nonuniformity of the material, the time period the
sample is energized, the magnitude and
polarity of the voltage, the time required
for the charge to build, material decay, and
specimen contour.
Several different types of insulation
resistance tests are performed when
troubleshooting electric motors. These
tests include an insulation spot test, a
dielectric absorption test, and an insulation step voltage test.
Insulation Spot Test
An insulation spot test is an insulation
resistance test that can be used to verify
the condition of the insulation over the
life of the motor. See Figure 3-2.
A spot test should be performed at regular intervals (every six months, for example) to track the changes in insulation
integrity. The test should also be performed
after a motor is serviced. The test measurement data can be recorded on a test graph
over time.
To perform an insulation spot test,
apply the following procedure:
An insulation multimeter can be used
to check insulation integrity between
commutator segments prior to rewinding.
1. Connect a megohmmeter to measure
the resistance of each winding lead
to ground. Record the readings after
60 sec.
2. Discharge the motor windings.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
23
NONCONTACT TEMPERATURE MEASUREMENT
1 mV DISPLAY
FOR EACH °F OR °C
DMM
Siemens
NONCONTACT
TEMPERATURE
PROBE
NONCONTACT TEMPERATURE PROBE
INFRARED
THERMOMETER
Siemens
INFRARED THERMOMETER
Figure 3-1. To measure the temperature of motor windings, use a noncontact
temperature probe connected to a DMM or an infrared thermometer.
24
INSULATION RESISTANCE TESTING
INSULATION SPOT TEST
DISCHARGE
MOTOR WINDINGS
2
TO GROUND
5k ,5W
RESISTOR
T8
TO GROUND
T5
T4
T1
CONNECT MEGOHMMETER
TO MEASURE RESISTANCE
OF EACH WINDING TO
GROUND AND RECORD
READING ON GRAPH
REPEAT STEPS 1 AND 2
EVERY SIX MONTHS
TEST
1
3
MEGOHMMETER
Figure 3-2. An insulation spot test is a test that checks motor insulation over the life
of the motor.
Interpret the results of the test graph
to determine the condition of the insulation. See Figure 3-3. Point A represents
the motor insulation condition when the
motor was placed in service. Point B
represents the effects of aging, contamination, etc., on the motor insulation. Point
C represents motor insulation failure.
Point D represents the motor insulation
condition after being rewound. Record
the lowest meter reading on an insulation
spot test graph if all readings are above
the minimum acceptable resistance. Service the motor if a reading does not meet
the minimum acceptable resistance. The
lowest reading is used because a motor is
only as good as its weakest point.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
25
INSULATION SPOT TEST GRAPH
1000
RESISTANCE (IN M )
500
A
B
MOTOR PLACED
IN SERVICE
100
EFFECTS OF AGING,
CONTAMINATION, ETC.
D
CONDITION AFTER
BEING REWOUND
50
10
5
C
0
1
2
3
4
MOTOR INSULATION
FAILURE
5
YEAR
Figure 3-3. Insulation spot test measurement data is recorded on a test graph
over time.
Dielectric Absorption Test
A dielectric absorption test is an insulation resistance test that checks the absorption characteristics of wet or contaminated
insulation. The test is performed over a
10 min period. See Figure 3-4.
To perform a dielectric absorption test,
apply the following procedure:
1. Connect a megohmmeter to measure
the resistance of each winding lead
to ground. Record the lowest meter
reading on a dielectric absorption
test graph if all readings are above
the minimum acceptable resistance.
Record the readings every 10 sec for
the first minute and every minute
thereafter for 10 min.
2. Discharge the motor windings.
Megohmmeters can be used for testing
defective motor winding insulation.
26
INSULATION RESISTANCE TESTING
DIELECTRIC ABSORPTION TEST
5k ,5W
RESISTOR
2
TO GROUND
T7
T8
T9
T4
T5
T6
T1
T2
T3
DISCHARGE
MOTOR
WINDINGS
TO GROUND
TEST
CONNECT MEGOHMMETER
TO MEASURE RESISTANCE
OF EACH WINDING TO
GROUND AND RECORD
READING ON GRAPH
1
MEGOHMMETER
Figure 3-4. A dielectric absorption test is a test that checks the absorption characteristics
of wet or contaminated insulation.
Interpret the results of the test graph to
determine the condition of the insulation.
See Figure 3-5. The slope of the curve
indicates the condition of the insulation.
Good insulation (curve A) indicates a
continual increase in resistance. Moist
or cracked insulation (curve B) shows
a relatively constant resistance. Service
the motor if a reading does not meet the
minimum acceptable resistance.
A polarization index is the ratio of the
insulation resistance of a machine winding calculated by using values obtained
from a 10 min measurement divided by
a 1 min measurement. The polarization
index is an indication of the condition of
the insulation. A low polarization index
(typically 1.5 or lower) indicates excessive moisture or contamination. Different
insulation classes will have different minimum acceptable polarization index values.
See Figure 3-6. Insulation classes
are based on the continuous operating
temperature of the application. For example, Class A insulation has a maximum operating temperature of 212°F,
Class B, 248°F, Class F, 293°F, and
Class H, 329°F.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
27
DIELECTRIC ABSORPTION TEST GRAPH
1000
500
CURVE A
(GOOD INSULATION)
(IN M )
RESISTANCE
100
CURVE B
(MOIST OR CRACKED
INSULATION)
50
10
5
1
2
3
4
5
6
7
8
9
10
TIME (IN MINUTES)
Figure 3-5. Dielectric absorption test measurement data is recorded on a test graph
over time.
MINIMUM ACCEPTABLE
POLARIZATION
INDEX VALUES
Insulation
Index Value
Class A
1.5
Class B
2.0
Class F
2.0
Class H
2.0
Figure 3-6. Different insulation classes
will have different minimum acceptable
polarization index values.
IEEE Standard 43-2000, Recommended
Practice for Testing Insulation Resistance
of Rotating Machinery, covers measurement
of polarization index testing.
For example, if the 1 min reading
of Class B insulation is 80 MΩ and the
10 min reading is 90 MΩ, the polarization
index is 1.125 (90 MΩ ÷ 80 MΩ = 1.125).
The polarization index is lower than the
minimum accepted values because the
insulation contains excessive moisture
or contamination.
Insulation Step Voltage Test
An insulation step voltage test is a test that
creates electrical stress on internal insulation cracks to reveal aging or damage not
found during other motor insulation tests.
This test is done by testing the insulation
at two or more voltages and comparing
the results. The insulation step voltage
28
INSULATION RESISTANCE TESTING
test is performed only after an insulation
spot test. See Figure 3-7.
3. Increase the megohmmeter setting
by increments of 500 V starting at
1000 V and ending at the high-end
voltage for a range of motors within
the system. Record each reading after
60 sec.
To perform an insulation step voltage
test, apply the following procedure:
1. Set the megohmmeter to 500 V and
connect to measure the resistance of
each winding lead to ground. Take
each resistance reading after 60 sec.
Record the lowest reading.
2. Place the meter lead on the winding
that has the lowest reading.
4. Discharge the motor windings.
Interpret the results of the test to
determine the condition of the insulation. See Figure 3-8. The resistance of
good insulation that is thoroughly dry
INSULATION STEP VOLTAGE TEST
4
TO GROUND
5k ,5W
RESISTOR
DISCHARGE
THE MOTOR
WINDINGS
T3
T2
T1
SET MEGOHMMETER
TO 500 V AND MEASURE
RESISTANCE OF EACH
WINDING TO GROUND
PLACE METER LEAD
ON WINDING WITH
LOWEST READING
TO GROUND
1
V
M
2
0M
00M
G
2500V
000V
0G
INCREASE METER
SETTING BY 500 V;
RECORD READING
ON GRAPH AFTER
60 SECONDS
500V
IR
PORT
00G
00k
T
0
k
ON/
OFF
FUNCTION
UP
SCROLL
DOWN
T
M
ENTER
TEST
3
Figure 3-7. An insulation step voltage test is a test that creates electrical stress on
internal insulation cracks to reveal aging or damage not found during other motor
insulation tests.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
(curve A) remains approximately the same
at different voltage levels. The resistance
of deteriorated insulation (curve B)
decreases substantially at different voltage levels.
The larger the insulation surface, the
lower the insulation resistance. The greater
the supply voltage to the motor, the thicker
the insulation requirements. A 50 HP,
230 V motor requires the same insulation
as a 1 HP, 230 V motor.
Caution: A megohmmeter uses very high
voltages during insulation resistance testing (up to 5000 V). Always follow recommended procedures and safety rules. After
performing insulation resistance tests with
a megohmmeter, connect the device being
tested to ground through a 5 kW, 5 W resistor if the megohmmeter does not include a
discharge function.
29
TROUBLESHOOTING CABLE
AND WIRE INSTALLATIONS
When troubleshooting cable and wire
installations, they should be disconnected
from panels and machinery to keep them
isolated. The conductors should be tested
against each other and against ground.
See Figure 3-9.
Cables and wires should be tested
and maintained on a three-year cycle at
a minimum. Insulation resistance tests
should be performed more frequently for
those systems that show deterioration of
insulation material. When performing
insulation tests on wire and cable, apply
the following procedure:
1. Inspect exposed sections of cables and
wires for physical damage. Replace or
repair sections that exhibit damage.
INSULATION STEP VOLTAGE TEST GRAPH
1000
500
CURVE A
(DRY INSULATION)
(I N M )
RESISTANCE
100
50
CURVE B
(DETERIORATED
INSULATION)
10
5
1
2
3
4
5
VOLTAGE (IN kV)
Figure 3-8. Insulation step voltage test measurement data is recorded on a test graph
over time.
30
INSULATION RESISTANCE TESTING
TROUBLESHOOTING CABLE AND WIRE INSTALLATIONS
METAL CONDUIT
TEST
CONDUCTOR
INSULATION
CABLE
INSULATION
CONDUCTORS
Figure 3-9. When troubleshooting cable and wire installations, the conductors should
be tested against each other and against ground.
2. Inspect cables and wires for proper
grounding, cable support, and termination. Terminate those sections that
are not properly terminated.
3. If cables and wires are properly
terminated, verify that neutrals and
grounds are properly terminated for
operation of protective devices.
4. Perform an insulation resistance
test on each conductor in the cable.
Apply 1000 VDC for 1 min (oneminute insulation resistance test)
to low voltage cables (1 kV or
less) and use a megohmmeter, or
insulation multimeter (IMM) to
measure the insulation resistance.
(Use a hipot tester to perform a DC
hipot test on cables ranging from
1 kV to 69 kV.)
Low-Voltage Cables
A cable length test should be performed
on newly installed low-voltage cables.
Knowing the actual length of low-voltage
cables is required because low-voltage
cables, like all conductors, have resistance
that reduces the amount of power passing
through them. The greater the length of a
cable, the greater the power reduction (attenuation) created by the cable. Because
the signals are transmitted at low power,
any reduction in power caused by a cable
must be kept to a minimum.
Cable length testers measure the length of
cables and also indicate the distance to a
cable fault by accessing the end of a cable
with two or more conductors.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
TROUBLESHOOTING
ELECTRICAL SWITCHGEAR
AND SWITCHBOARDS
Electrical switchgear and switchboards
are freestanding assemblies of metalenclosed sections containing circuit
breakers and/or fused disconnect switches. They also contain bus bars, cable
termination points, backup protection
devices, and various forms of controls and
instrumentation. Switchgear or switch-
31
boards are used to distribute utility power
via busways to transformers, panelboards,
and motor starters. See Figure 3-10. An
assembly may be part of a load center,
substation, or distribution board.
Switchgear and related assemblies
should be tested and maintained at a minimum of once a year. Inspections should be
performed more frequently if equipment
is in an environment that has excessive
dirt or moisture. Per plant procedures and
manufacturers recommendations, the best
SWITCHGEAR POWER DISTRIBUTION
OUTDOOR FEEDER BUSWAY
FEEDER BUSWAY
480 V FROM
UTILITY
480 V
SWITCHGEAR/
SWITCHBOARD
PANELBOARD
TRANSFORMER
TRANSFORMER
480 V
480 V
120 V
120 V
480 V
480 V
PANELBOARD
PANELBOARD
MOTOR STARTER
Figure 3-10. Switchgear or switchboards are used to distribute utility power via busways
to transformers, panelboards, and motor starters.
32
INSULATION RESISTANCE TESTING
results are achieved when the following
actions are applied:
• Inspect electrical sections of switchgear for damage. Repair or replace
damaged sections.
• Inspect all bus connections for high
resistance by using an insulation multimeter. Repair or replace sections that
are not in compliance.
• Inspect insulation material for physical damage or contaminated surfaces.
Clean or repair any insulation that is
not in compliance.
• Perform insulation resistance tests on
each bus section, phase-to-phase, and
phase-to-ground by using a megohmmeter or insulation multimeter. Typical
switchgear insulation resistance values
range from 50 MΩ to 20,000 MΩ. Insulation resistance values that are less
than the equipment manufacturer’s
recommended minimum resistance
value should be investigated for additional problems.
• Always perform tests on deenergized
equipment.
Although standard insulation resistance specifications can be applied when
troubleshooting most motors and other
electrical devices, certain situations require higher resistance specifications to
be applied. Most stated specifications list
the maximum amount of acceptable leakage current and not the actual resistance.
For example, a 3-wire handheld electrical appliance or tool must be insulated
enough to allow no more than 0.75 mA
(0.00075 A) of leakage current to flow
through the exposed parts to ground.
Typical resistance values are one-third
higher than the 1 MΩ per 1 kV industry
standard. Medical equipment and electrical devices rated as “double-insulated”
have much higher insulation ratings
than ordinary equipment. Miscellaneous
equipment includes two-prong Category II power cord devices, three-prong
Category I power cord devices, and drytype transformers.
Insulation resistance values vary with temperature and the amount of moisture in the
insulation. The temperature and humidity
should be recorded at the time the insulation
test readings are taken.
TROUBLESHOOTING
MISCELLANEOUS
ELECTRICAL DEVICES
Two-Prong Category II Power
Cord Devices
Miscellaneous equipment includes such
equipment as portable electrical devices,
double-insulated tools, extension cords,
transformers, and any electrical device
that receives power through a separate
power conductor. Insulation resistance
testing is performed on such equipment
as part of regular maintenance programs
as well as for standard OSHA and
NEC® compliance.
A two-prong Category II power cord
device is a device that has only two conductors extending from it, one hot and one
neutral. Two-prong devices do not have
a third ground (green wire) prong on the
power cord. Certain two-prong devices
are classified as double-insulated.
A double-insulated device is an electrical product designed so that a single
ground fault cannot cause a dangerous
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
electrical shock through any exposed
sections of the product that could come
in contact with an electrician. Doubleinsulated devices include not only the
standard insulation used on conductors
but also extra insulating material between
the energized parts of the device and the
parts that can be contacted.
When troubleshooting these devices
for insulation resistance, leakage current
is measured from the exposed metal parts
to ground. See Figure 3-11. To test a twoprong Category II power cord device for
insulation resistance, apply the following
procedures:
1. If the device has no exposed metal,
wrap metal foil on the exposed
plastic parts (such as the handles).
The metal foil simulates a wet hand
contacting the electrical device.
2. Ensure that the electrical device
being tested is not plugged into a
power source.
3. Connect the ground test lead to
the metal foil wrap on the tool
under test.
4. Connect the voltage test lead to the
neutral (larger) blade on the tool’s
power cord.
5. Use the insulation resistance tester to
supply the test voltage.
6. Measure the amount of leakage current in mA and record the readings.
Typical specified maximum leakage
current for a double-insulated, twoprong Category II device is typically
0.25 mA (0.00025 A).
When leakage current exceeds the
specified limit, a three-prong power
cord must be used. The ground (green)
wire is added to carry the leakage
current to ground by providing a low
33
impedance (resistance) path from all
non-current-carrying parts to earth
ground. Equipment used in the medical
field will have an even lower acceptable
maximum leakage current limit.
Three-Prong Category I Power
Cord Devices
A three-prong Category I power cord device is a device that has three conductors
extending from it, one hot, one neutral,
and one ground. Any leakage current
will flow through the ground (green)
conductor back to ground during normal
operation. The ground conductor prevents
the exposed metal parts of the electrical
device from becoming energized to the
point of causing an electrical shock.
See Figure 3-12. To test a three-prong
Category I power cord device for insulation resistance, apply the following
procedure:
1. Ensure that the electrical device being tested is not plugged into a power
source.
2. Connect the ground test lead to a metal
portion of the equipment under test,
such as a motor housing.
3. Connect the voltage test lead to the
neutral (larger) blade on the tool’s
power cord.
4. Measure the amount of leakage current
in mA and record the readings.
The more power a load requires, the larger
the amount of current flow. For example, a
10 HP motor draws approximately 28 A when
wired for 230 V. A 20 HP motor draws approximately 54 A when wired for 230 V.
34
INSULATION RESISTANCE TESTING
TWO-PRONG CATEGORY II POWER CORD DEVICES
0
2
1
3
4
5
6
MICROAMPERES
DC
MEASURE AMOUNT OF LEAKAGE
CURRENT AND RECORD READING.
(0.25 mA MAXIMUM LEAKAGE
CURRENT ALLOWED)
HIPOT
TESTER
5
STABILIZATION
OUT
1
0
2
3
OUTPUT
15 KV
MEGOHMMETER
IN
4
5
3
0
0
6
9
3
4.5
1.5
MICROAMPERES
DC
12
6
CAUTION
HIGH
VOLTAGE
15
USE INSULATION
RESISTANCE TESTER TO
SUPPLY TEST VOLTAGE
7.5
KILOVOLTS
DC
120 V
VOLTAGE
CONTROL
CAL
CAL
I I
MULTIPLIER
3
60
AC ON OUTPUT ON
I
I I
I I
EXT
INST
40
CAL
I I
MULTIPLIER
80
20
0
OFF
OFF
100
CONNECT GROUND
TEST LEAD TO METAL
FOIL WRAP
VOLTAGE CONTROL
SET ON 100
DOUBLE-INSULATED
ELECTRICAL DEVICE
WITHOUT GROUND
PLASTIC HOUSED
TOOL REQUIRES
METAL FOIL WRAP
1
CATEGORY II
ELECTRICAL
DEVICE
TWO-PRONG
POWER CORD
4
DEVICE NOT PLUGGED
INTO POWER SOURCE
CONNECT VOLTAGE
TEST LEAD TO
NEUTRAL PRONG
2
Figure 3-11. A two-prong Category II power cord device has only two conductors
extending from the device, one hot and one neutral, and does not have a third ground
prong on the power cord.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
35
THREE-PRONG CATEGORY I POWER CORD DEVICES
0
2
1
3
4
5
4
MICROAMPERES
DC
MEASURE AMOUNT OF LEAKAGE
CURRENT AND RECORD READING
(3.5 mA MAXIMUM LEAKAGE
CURRENT ALLOWED)
HIPOT
TESTER
STABILIZATION
OUT
0
1
2
3
OUTPUT
15 KV
MEGOHMMETER
IN
4
MICROAMPERES
DC
5
3
0
0
6
9
3
4.5
1.5
12
6
KILOVOLTS
DC
CAUTION
HIGH
VOLTAGE
15
7.5
120 V
VOLTAGE
CONTROL
CAL
CAL
I I
40
CAL
I I
MULTIPLIER
60
AC ON OUTPUT ON
I
I I
I I
EXT
INST
MULTIPLIER
80
20
0
OFF
OFF
100
VOLTAGE CONTROL SET ON 100
2
CONNECT GROUND TEST LEAD
TO METAL PORTION OF
EQUIPMENT UNDER TEST
ELECTRICAL DEVICE
WITH GROUND PLUG
CATEGORY I
ELECTRICAL
DEVICE
1
3
ELECTRICAL DEVICE
THREE-PRONG POWER
CORD NOT PLUGGED
INTO POWER SOURCE
CONNECT VOLTAGE
TEST LEAD TO
NEUTRAL PRONG
GROUND
PRONG
Figure 3-12. A three-prong Category I power cord device has three conductors
extending from the device, one hot, one neutral, and one ground (green wire).
36
INSULATION RESISTANCE TESTING
The typical specified maximum leakage current for a three-prong category I
device is typically 0.75 mA (0.00075 A)
for handheld electrical devices such as
disc grinders, and 3.5 mA (0.0035 A) for
non-handheld electrical devices such as
floor buffers, electric motors, small drill
presses, and air compressors.
A reference multimeter can be used to
take resistance measurements from 2 Ω to
20 MΩ, as well as more accurate voltage,
current, and temperature measurements
than a standard handheld meter.
General Electric Company
Autotransformers are used in starting
rotating machinery such as synchronous
and induction motors.
Dry-Type Transformers
A dry-type transformer is a transformer
that uses a non-liquid material for insulation. Dry-type transformers can be
constructed as either toroidal (doughnutshaped) or laminated. Toroidal transformers typically have copper wire wrapped
around a cylindrical core so the magnetic
flux within the coil does not leak out, has
good coil efficiency, and has little effect
on other components. Laminated transformers contain laminated-steel cores.
The steel laminations are insulated with
a nonconducting material, such as varnish
or a high-temperature polymer, and then
formed into a core that reduces electrical
losses. Dry-type transformer maintenance
and testing should be performed and
recorded, starting with the initial installation of the transformer.
An IMM set to measure resistance can
be used to check for open circuits in coils,
short circuits between primary and secondary coils, or coils shorted to the core
without power applied to the transformer.
See Figure 3-13.
Open Circuits in Coils. The resistance
of each coil is checked with a DMM.
The winding is open and the transformer
is bad if any of the coils show an infinite
resistance reading. Note that very low
resistance readings do not indicate a short,
but rather the resistance of the wire.
Short Circuits between Primary and
Secondary Coils. A check for short circuits should be made between the primary
and secondary coils of the transformer.
Checking for short circuits between the
primary and secondary coils should be
performed with a megohmmeter.
Chapter 3 — Troubleshooting with Insulation Resistance Test Instruments
37
TESTING TRANSFORMERS
TRANSFORMER
NORMAL
RESISTANCE
READING
INDICATES
GOOD COIL
PRIMARY
WIRES
SECONDARY
WIRES
IMM
OPEN CIRCUITS IN COILS
INFINITE READING
WHEN CHECKING FOR
SHORT BETWEEN COILS
INDICATES NOT SHORTED
TO EACH OTHER
PRIMARY
WIRES
TRANSFORMER
SECONDARY
WIRES
IMM
SHORT CIRCUIT BETWEEN
PRIMARY AND SECONDARY COILS
PRIMARY
WIRES
TRANSFORMER
IMM
INFINITE READING WHEN
CHECKING FOR SHORT
BETWEEN WINDING AND
BODY INDICATES GOOD
TRANSFORMER
SECONDARY
WIRES
COILS SHORTED TO CORE
Figure 3-13. An IMM set to measure resistance can be used to check for open circuits
in coils. When testing for short circuits between primary and secondary coils, or for
coils shorted to the core without power applied to the transformer, use the insulation
test function of the IMM.
38
INSULATION RESISTANCE TESTING
Coils Shorted to Core. A resistance
check is made from each transformer coil
to the core of the transformer. All coils
should show an infinite resistance reading
to the core. The transformer should not be
used if a resistance is shown between any
coil and the core.
Transformer Maintenance. Transformer manufacturers provide recommended maintenance procedures and
schedules for their equipment, but the
actual performance of a specific transformer should determine the frequency
of the maintenance and troubleshooting to
be performed.
The majority of transformer failures
are caused by poor maintenance. Transformers should be tested and maintenance
performed at least once annually. When
performing insulation resistance tests on
dry-type transformers, apply the following procedures:
1. Visually inspect for physical damage: cracked insulation, defective
wiring, tightness of connections, and
dirt/moisture on the coil.
2. Verify proper core and equipment
grounding.
3. Use a megohmmeter or insulation
multimeter to perform insulation
resistance tests on winding to winding or windings to ground. Perform insulation resistance tests for
each winding.
Typical transformer insulation resistance values range from 500 MΩ to
25,000 MΩ. See Figure 3-14. Insulation
resistance test readings for transformer
windings should be within 1% of adjacent
windings.
TRANSFORMER INSULATION
RESISTANCE VALUES
Transformer
Coil Rating*
Minimum
Test
Voltage†
Minimum
Insulation
Resistance‡
0 - 600
1000
500
601 - 5000
2500
5000
5001 - 15,000
5000
25,000
* in VAC
†
in VDC
‡ in
MΩ
Figure 3-14. Typical transformer insulation
resistance values range from 500 MΩ to
25,000 MΩ.
When inspecting small (600 V or less)
dry-type transformers, apply the following procedures:
1. Visually inspect for physical damage: cracked insulation, defective
wiring, tightness of connections, and
dirt/moisture on the coil.
2. Verify proper core and equipment
grounding.
3. Clean the unit prior to making any
tests.
4. Use a megohmmeter or insulation
multimeter to perform insulation
resistance tests on winding to winding or windings to ground. Perform insulation resistance tests for
each winding.
To help prevent electrical problems that
cause equipment damage, neutral conductors
should be the same size as, or larger than,
hot conductors.